just need a clarification..as we know that when any any plate subduct under other plate,formation of mountains takes place and due to to melting of subducted plate in aesthenosphere it leads to formation of volcanoes behind mountains...same process had happend in the formation of himalayas but why there are no volcanoes behind himalayas but a plateau of tibet?

the process you are talking about happens when oceanic plate subducts under continental plate. but in case of Himalayas the two plates that collided both were continental plate hence no volcano formation in Himalayas.

Question 65
At a particular section of a reaction turbine, the diameter of the blade is
1.8 m, the velocity of flow of steam is 49 m/s and the quantity of steam flow
is 5.4 m3/s. The blade height at this section will be approximately:
(a) 4 cm
(b) 2 cm
(c) 1 cm
(d) 0.5 cm

Question 66
Assertion (A): During the operation of a steam turbine, it is necessary to maintain
the moisture content of steam below 10%. Hence, the steam quality at turbine exit
must be greater than 0.9.
Reason (R): The precaution has to be taken in order to prevent corrosion and the
consequent damage to the turbine.
(a) Both A and R are individually true and R is the correct explanation of A
(b) Both A and R are individually true but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
Ans: c

Question 67
In a surface condenser used in a steam power station, under cooling of
condensate is undesirable at this would
(a) Not absorb the gases in steam
(b) Reduce efficiency of the plant.
(c) Increase the cooling water requirements.
(d) Increase thermal stresses in the condenser
Ans: C

Question 68
If in an impulse turbine designed for free vortex flow, tangential velocity
of steam at the root radius of 250 mm is 430 m/s and the blade height is 100
mm, then the tangential velocity of steam at the tip will be
(a) 602 m/s(b) 504 m/s
(c) 409 m/s
(d) 307 ms

Question 69
Consider the following statements: In an impulse turbine
1. The relative velocity of steam at inlet and outlet of moving blades are
equal.
2. The moving blades are symmetrical
3. The outlet area of the moving blades is smaller than the inlet area.
Of these statements:
(a) 1, 2 and 3 are correct
(c) 1 and 2 are correct
(b) 2 and 3 are correct
(d) 1 and 3 are correct
Ans: c